Solid state power switches used as power control devices tend to fail in shorted states, which is often unacceptable in power switching applications. Various fault detection circuits for detecting such short circuits in power switching applications are known. In one such technique, a second solid state power switch device is used to turn off the current when a primary solid state power switch fails to respond. This method is not failsafe, since there is no way to know if one of the switches has failed closed.
In another technique, a mechanical fuse or circuit breaker is added in series with the solid state power switch device. This technique is based on the assumption that when the solid state power switch device is shorted, so is the load. This is not always the case. Therefore, this technique fails to detect a shorted solid state power switch where the power switch is shorted but the load is not.
In another technique, a thermal fuse is placed in close proximity to a power control device such that the fuse opens the circuit when the solid state power switch carries the full load current, thereby overheating the fuse, or in the event that the temperature of the power switch exceeds the fuse's temperature rating. This technique is effective even where the power switch is shorted but the load is not. However, this technique is not failsafe in that it requires the power control device itself to exceed the fuse's temperature or current rating in order for the circuit to be opened.
In another technique, the output current is compared with the switch control voltage, and if there is output current but the switch control voltage is zero, then a thermal fuse is heated until it opens the circuit. However, this technique must be carefully adapted for different size loads and does not provide user-resettable  capabilities. Examples of current techniques are disclosed in various patents, such as those described below, each of which is incorporated herein by reference. The current techniques, however, do not provide a fault detection and switching circuit and method of operation for use with a power control device to detect an abnormal open or abnormal closed state in the power control device.
U.S. Pat. No. 5,224,006, “Electronic circuit breaker with protection against sputtering arc faults and ground faults”, discloses a circuit breaker that bandwidth limits a sensed rate of change of current, di/dt, in the neutral conductor of a protected electrical system and fullwave rectifies the bandwidth limited di/dt signal for comparison with an instantaneous trip reference signal.
U.S. Pat. No. 5,381,296, “Short circuit limiting protector”, discloses a current limiting short-circuiting protector which utilizes an electronic and thermal feedback principle for achieving precise and rapid release of the protector, which can be readily reset.
U.S. Pat. No. 5,536,980, “High voltage, high current switching apparatus”, discloses a high voltage, high current DC switch having a single pole, double throw relay and a solid state power switch such as an IGBT or MOSFET transistor.
U.S. Pat. No. 5,691,869, “Low cost apparatus for detecting arcing faults and circuit breaker incorporating same”, discloses an analog arcing detector and a circuit breaker incorporating such a detector that provides a variable response time to arcing faults based upon the amplitude of the arcing current.
U.S. Pat. No. 5,703,463, “Methods and apparatus for protecting battery cells from overcharge”, discloses a method and apparatus for protecting at least one battery cell from overcharge that utilizes a combination of at least one MOSFET switch and a separate fail-safe element such as a fuse or circuit breaker.
U.S. Pat. No. 5,767,724, “Electronic clamping circuit”, discloses an electronic clamping circuit that includes a pair of diodes connected in series, both having the same bias, which are shunted across a feedback path of a transimpedance amplifier circuit.
U.S. Pat. No. 5,818,673, “Electric power distribution system having fault bypass feature”, discloses an electric power distribution system comprising: a plurality  of electric power distribution portions for distributing electric power to a plurality of various loads; and a power supply portion connected to at least any one of the plural electric power distribution portions such that the plural electric power distribution portions are able to supply electric power through at least two systems of electric lines.
U.S. Pat. No. 6,031,743, “Fault isolation in a redundant power converter”, discloses a system for isolating faults in a redundant power converter including a first switch at an input of the redundant power converter for protecting the redundant power converter from a high input voltage, and a second switch coupled within the redundant power converter prior to an output capacitor, wherein efficiency of fault isolation is improved.
U.S. Pat. No. 6,114,672, “PTC-element, protective device and electric circuit board”, discloses a PTC-element to be used as a protective device against overcurrent or overvoltage, of which the volume resistivity does not substantially differ before and after a high temperature processing, such as reflowing of solder, and can be preserved even after having been subjected to repeated tripping, which contains a non-conductive crystalline polymer and a particulate conductor material dispersed in the non-conducting crystalline polymer, wherein the particulate conductor material includes electro-conductive particles having irregular surface contours.
U.S. Pat. No. 6,157,091, “Power supplying apparatus for a vehicle and an intensive wiring apparatus”, discloses a specific loop wiring topology intended to facilitate determination of the location of short circuit abnormalities in a power supply system.
U.S. Pat. No. 6,421,214, “Arc fault or ground fault detector with self-test feature”, discloses a self-testing arc fault or ground fault detector that includes arc fault detecting circuitry and components. The detector includes a testing circuit which tests at least part of the circuitry and components and generates a recurring signal when the test completes successfully. If the test does not complete successfully, the signal is lost. This loss of signal is signaled by an indicator connected to the testing circuit. In one version, the loss of signal activates a circuit interrupter which disconnects the load side of the detector from the line side. 
U.S. Pat. No. 6,606,228, “Fault detection circuit for use with a power control device”, discloses a fault detection circuit that includes a controllable switch coupling a power source to a load. Control circuitry determines if the switch is in the proper conduction state based on a switch control signal and a signal indicative of power delivered to the load. If the switch is determined as improperly closed (conducting), the control circuitry diverts energy delivered to the load through fuse circuitry, thereby blowing a fuse and decoupling the load from the power source.
U.S. Pat. No. 6,744,650, “Method for controlling a matrix converter”, discloses a method for controlling a matrix converter which includes nine bi-directional circuit-breakers arranged in a 3 by 3 switch matrix.
U.S. Pat. No. 6,917,186, “Monitoring and control for power electronic system”, discloses a control method and arrangement that monitors the condition and operating parameters of a power electronic system having power electronic devices and responds to various detected abnormalities to optimize operation of the power electronic system.
U.S. Pat. No. 6,927,963, “Fault detection circuit for use with a power control device”, discloses a fault detection circuit that includes a controllable switch coupling a power source to a load. Control circuitry is provided that determines if the switch is in the proper conduction state based on a switch control signal and a signal indicative of power delivered to the load. If the switch is determined as improperly closed (conducting), the control circuitry diverts energy delivered to the load through fuse circuitry, thereby blowing a fuse and decoupling the load from the power source.
U.S. Pat. No. 6,943,558, “System and method for remotely detecting electric arc events in a power system”, discloses a system for remotely detecting and locating damaged conductors.
U.S. Pat. No. 6,950,764, “Arrangements to detect and respond to disturbances in electrical power systems”, discloses a control arrangement and method for detecting and responding to disturbances in electrical power systems. When an integration based on a comparison of actual voltage of a source and a reference voltage exceeds a predetermined value, the source is considered unreliable. 
U.S. Pat. No. 6,952,335, “Solid-state DC circuit breaker”, discloses a high-speed, solid-state circuit breaker capable of interrupting high DC currents without generating an arc. The high-speed, solid-state DC circuit breaker uses an emitter turn-off (ETO) thyristor as the switch.
U.S. Pat. No. 7,102,252, “Multi-function solid-state switch”, discloses a dual solid state switch architecture with a plurality of control/monitor ports, selected ones of which are used to control the operation of a pair of power MOSFETs current flow paths through which are coupled to prescribed ones of a plurality of input/output ports that are adapted to be coupled to a circuit path containing a load and an AC or DC power source. Selected others of the control/monitor ports are used to monitor current delivered to the load. A leakage current by-pass resistor is connected between selected input/output ports, to which a neon tube may be connected for indicating the switching on of an AC source. This resistor serves to provide a bypass path for leakage current through the MOSFETs, so as to prevent the neon tube from being erroneously illuminated, when the MOSFETs have been turned off.
U.S. Pat. No. 7,133,267, “Arc detection circuit”, discloses an arc detector comprising a voltage generator for detecting a voltage and an integrator for integrating said voltage with respect to time and generating an output signal corresponding to said integration.
U.S. Pat. RE39,710, “System for providing assured power to a critical load”, discloses a power system for supplying uninterrupted electrical power to one or more critical loads, comprising one or more fuel cell power plants to provide one substantially continuous source of power, and a utility grid to provide another source of power.
U.S. Patent Publication 20040156154, “Arc fault detection for SSPC based electrical power distribution systems”, discloses an apparatus and a method for detecting arc faults in an electrical power distribution system of a vehicle. The apparatus includes: a load current input arranged to obtain a measurement of current being supplied to a load via a solid state power switching device of the electrical power distribution system; and an arc fault detector arranged to compare the measured load current with a load signature, the load signature being a function of characteristics of the  load being supplied current via the solid state power switching device and operating conditions of the solid state power switching device.
Modern aircraft typically have many critical systems that are electrically controlled. Failure of a power switch can lead to hazardous conditions, and so the ability to monitor power switching systems and detect faults can be especially important in aircraft applications. The following patent applications, each of which is incorporated herein by reference, describe various aircraft instrumentation and operation methods and apparatuses in connection with the present invention can be useful:    U.S. provisional application 60/853,712, filed Oct. 23, 2006;    U.S. patent application Ser. No. 11/311,060, filed Dec. 19, 2005;    U.S. patent application Ser. No. 11/875,813, filed Oct. 19, 2007, Aircraft Emergency Handling;    U.S. patent application Ser. No. 11/875,815, filed Oct. 19, 2007, Backup Electrical Power System for Solid-State Aircraft Power Distribution Systems;    U.S. patent application Ser. No. 11/875,816, filed Oct. 19, 2007, Aircraft Electrical System Evaluation;    U.S. patent application Ser. No. 11/875,818, filed Oct. 19, 2007, Aircraft Exhaust Gas Temperature Monitor;    U.S. patent application Ser. No. 11/875,819, filed Oct. 19, 2007, Variable Speed Flap Retraction and Notification.
Accordingly, there is a need for methods and apparatuses that provide for testing of power switches without requiring destructive elements such as fuses, and that allow switch integrity testing before power activation and during operation.